Dockerfile.e2e: pin to debian stretch variant for now

[thaJeztah]

We saw a change in behaviour for some tests in moby/moby, and also wondering if this is related to flakiness of the TestSigProxyWithTTY test; #2009 (comment)https://jenkins.dockerproject.org/job/docker/job/cli/job/PR-2009/11/execution/node/28/log/

01:14:47 === Failed
01:14:47 === FAIL: e2e/container TestSigProxyWithTTY (5.06s)
01:14:47 proxy_signal_test.go:35: timeout hit after 5s: expected status running != 01:14:47 01:14:47 

[thaJeztah]

Ok, that didn't make a difference for that test, unfortunately 😞

[codecov-io]

Codecov Report

Merging #2013 into master will not change coverage.
The diff coverage is n/a.

@@ Coverage Diff @@## master #2013 +/- ##
=======================================
Coverage 56.79% 56.79% =======================================
Files 311 311 Lines 21836 21836 =======================================
Hits 12402 12402 Misses 8519 8519 Partials 915 915

[thaJeztah]

Flaky test was fixed, so I think we should be ok without this